Passive optical networks (PONs) can be utilized to provide data from one or more user nodes (e.g., an optical network unit [ONU], optical network terminal [ONT], etc.) to a central node (e.g., a central office, such as or including an optical line terminal [OLT]) using optical signal media (e.g., a fiber optic cable, a fiber optic link, etc.). In some PONs, time-division multiplexing (TDM) is used so that multiple bit streams or data signals (e.g., data packets) are alternately transmitted by one or more ONUs over a single communication channel. That is, in PONs employing TDM, a first ONU can transmit optical data during an allocated or predetermined time slot, and a second ONU can transmit optical data over the same media in the next allocated or predetermined time slot.
For a data packet to be accurately received by the OLT (e.g., including a burst mode optical receiver), a decision threshold must be set. That is, based on the amplitude or common-mode voltage of the received data signal, a decision threshold is set, where data having an amplitude above the decision threshold are considered to have a binary logic high state, and data having an amplitude below the decision threshold are considered to have a binary logic low state. For the decision threshold to be set correctly and/or accurately, the common-mode (or average) voltage of the incoming signal must be known.
In some embodiments, an ONU may be located at a first distance from an OLT, and provide data having a first common-mode voltage or amplitude A1 to the OLT. A second ONU may be located at a further, second distance from the OLT, and provide data having a second common-mode voltage or amplitude A2 to the OLT. In such circumstances, amplitude A2 is generally less than amplitude A1. Stated differently, optical packets received by the OLT from different ONUs may have different amplitudes (e.g., amplitudes A1 and A2). Thus, use of a common-mode voltage (e.g., VCM) established for the first packet may result in inaccurate reception of the second packet, since the amplitudes of the received packets are not necessarily equal (e.g., A2 may be significantly less than A1).
Specifically, as illustrated in graph 10 of FIG. 1, a first data signal 20 having an amplitude A1 and a common-mode voltage VCM is received by an optical receiver (not shown). At time t1, the first data signal 20 is no longer received, so the common-mode voltage VCM (and thus, the voltage threshold 25) decreases. At time t2, a second data signal 30 having an amplitude A2 less than A1 is received. However, the amplitude A2 is also less than VCM (and the voltage threshold 25) at time t2. The VCM decay after time t1 (e.g., as represented by voltage threshold 25) represents the discharge of a stored charge in an RC circuit in the receiver. As can be seen in FIG. 1, voltage threshold 25 has a value greater than A2 for data signal 30 values initially received after time t2, in which case a potential problem arises.
More specifically, the voltage threshold for first data signal 20 is set equal to the common-mode voltage VCM (e.g., using the RC circuit in the receiver). As discussed above, at time t1, the common-mode voltage VCM (and thus, the voltage threshold 25) begins to decrease, and continues to decrease after time t2. However, data (e.g., a data packet) in data signal 30 received just after time t2 is processed while the voltage threshold 25 is above the common-mode voltage of the second data signal 30. This relatively long decay in the voltage threshold 25 is caused by the RC time constant of the RC circuit in the receiver. Such RC circuitry may not be capable of processing certain data signals having different amplitudes sufficiently quickly. Thus, optical receiver circuitry using voltage threshold 25 for the data signal 30 transmitted by the second ONU may result in some of the data signal 30 being identified as having a low binary logic state, even when it does not.
Stated differently, since the peak amplitude (e.g., A2) of the second data signal 30 is less than the VCM of the first data signal 20, a portion of the data (e.g., a header for data signal 30) transmitted by the second ONU may always be below the voltage threshold 25, regardless of its actual value. The data arriving before time t3 therefore may be erroneously considered to have a low binary logic state, although the data would be correctly identified as having a high binary logic state once the voltage threshold 25 decreases to the common-mode voltage (e.g., the average voltage) of the second data signal 30.
Additionally, in most PONs having time-multiplexed signal transmission, data packets are transmitted close together to maximize bandwidth efficiency. Thus, the receiver must quickly determine the presence of and/or a decision threshold voltage for data signals having different common-mode voltages. However, current optical receivers employing feedback configurations to determine an optical data signal decision threshold may not be capable of processing such high bandwidth data signals sufficiently quickly. For example, in some optical receivers using feedback configurations, the sequence of steps for determining the common-mode voltage and decision threshold includes converting the received optical data signal to an electrical signal, amplifying the electrical signal, comparing the amplified electrical signal to a predetermined decision threshold, providing a feedback signal to an input of the amplifier, then adjusting the predetermined decision threshold based on the feedback signal. Such a configuration, however, increases data processing time, and data at the beginning of the data packet can be erroneously processed (e.g., treated as all binary logic “0”s) during the time that the decision threshold is being determined. Thus, the optical and optoelectronic networking industries seek optical and/or optoelectronic receivers and/or transceivers capable of quickly and accurately detecting a new data signal and determining a decision threshold for data signals received from multiple ONUs at various amplitudes.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.